%开根号
a=-8;
p=[1,0,0,-a];
R=roots(p);

MR=abs(R(1));
t=0:pi/20:2*pi;
x=MR*sin(t);
y=MR*cos(t);
plot(x,y,'b:'),grid on
hold on
plot(R(2),'.','MarkerSize',30,'Color','r')
plot(R([1,3]),'O','MarkerSize',15,'Color','b')
axis([-3,3,-3,3]),axis square
hold off
